1. Field of the Invention
The present invention relates to an imaging element such as CMOS (Complementary Metal Oxide Semiconductor) image sensor and a camera system.
2. Description of the Related Art
In recent years, vigorous measurement and imaging microscopic light and fluorescence emitted from biological bodies have been conducted in the fields of medicine and biotechnology.
Such imaging requires far higher sensitivity and lower noise than available ordinary imagers. An EM-CCD (Electron-Multiplying Charge-Coupled Device) is, for example, used that is capable of multiplying analog charge.
In the meanwhile, Japanese Patent Laid-Open No. 1995-67043 proposes an imaging element using time division photon counting.
This technique is designed to determine the presence or absence of a photon incident upon a photodiode for each given period as a binary value and compile the results obtained by repeating this process a plurality of times to obtain two-dimensional imaging data.
That is, this imaging element senses a signal from the photodiode every given period. So long as one or more photons are incident during that period, the counter connected to each of pixels is incremented by one irrespective of the number of incident photons.
When the frequency of incidence of photons upon the photodiode is random along the time axis, the actual number of incident photons and the count are consistent with the Poisson distribution. Therefore, when the frequency of incidence of photons is low, the actual number of incident photons and the count are in an approximately linear relationship. On the other hand, when the frequency of incidence of photons is high, the relationship can be corrected in an across-the-board manner.
Such an imaging element is particularly suited for imaging microscopic light thanks to its capability to completely eliminate read noise.
Such photon counting is generally implemented by charge multiplication.
For example, Japanese Patent Laid-Open No. 1995-67043 assumes that an avalanche diode is used to multiply the charge. An avalanche diode converts a photon incident upon the photo-receiving surface into a photoelectron and further accelerates the photoelectron across a high voltage to repeat the generation of secondary electrons by collision, thus multiplying the signal charge.
This provides a signal whose level is sufficiently large for detection of an incident photon.
An imaging element using photon counting is hardly subject to variations in sensitivity from one device to another. Therefore, an imaging surface can be formed by arranging such imaging elements.
Such an imaging element is expected to be used in a variety of applications including ultra-low exposure X-ray radiography by using it in combination with a scintillator.